Telephone



Oct. 18,1927. 1,645,773

B. ca. POHLMANN TELEPHONE OriginalF'iled Dec. 22. 21

Patented Oct. 18, 1927.

UNITED 'STATES PATENT OFFICE.

BRuNo GERHARD PQHLMANN, or SIEMENSSTADT, NEAR BERLIN, GERMANY, AS-

sreNon T SIEMENS & HALSKE, AKTIENGESELLSCHAFT, or SIEMENSSTADT, NEAR BERLIN, GERMANY, A conrona'rron on GERMANY.

TELEPHONE.

Application filed December 22, 1921, Serial In telephone installations with amplifiers it has hitherto been the practice, to select the natural frequency of the secondary winding of the input transformer so that 'it corresponds to the mean acoustic frequency, while the primary side is adapted to the resistance of the transmitter for this fre uency. By this means the greatest amp ification is obtained.

One disadvantage of such an arrangement is that, owing to the fact that the im edance of the input transformer is depen cut on the frequency, the amplification is different for the individual frequencies of the total range, the maximum lying at the mean acoustic frequency corresponding to the natural frequency of the secondary winding,

While lines, more particularly cables, are

selective as regards the lower frequencies as compared with the higher ones.

This can be overcome by compensating the detrimental effect on the higher frequencies, by selecting the natural frequency of the secondary winding of the transformers such that it is within the range of higher acoustic frequencies. The resulting flattening of the amplification curve by this means would be by no means perfect, as the damping curve of the line and the amplification curve cannot be brought into agreement by this means alone.

The invention solves the problem of obtaining a closer approximation of the course of the amplification curve to the damping curve. According to the invention this is effected 0n the one hand by placing the natural frequency of the secondary transformer winding within the range of higher acoustic frequencies and on the other hand by making the course of the amplification curve flatter or steeper according to the form of the damping curve. The amplification curve may be flattened,.for instance, by placing an impedance in parallel with the primary or secondary winding of the transformer.

The form of the amplification curve however, may be controlled in. a much simpler manner and more effectively by a suitable selection of the number of turns and consequently of the impedance of the primary winding. By this means it becomes possible, not only to flatten the amplification curve,

No. 524,259, and in Germany December 29, 1920. Renewed March 5, 1927.

but also to make the rising side of the curve steeper, if required as will be seen from the following.

In the case of input transformers in than mionic tube amplifying arrangements the impedance of the primary winding is a maximum for the natural frequency of the secondary winding, and the terminal voltage, occurring in the former and caused by the alternating current voltage present in the line, is also a maximum for that frequency.

Thus, by selecting a higher natural frequency for the secondary coil, the maximum of amplification is displaced towards the higher frequenc and the range of acoustic frequencies is isplaced wholly or. almost wholly into the ascending side of the amplification curve. The course of this ascending side of the curve should be as nearly as possible parallel to the damping curve, so that the remaining damping or attenuation is the same for all frequencies.

For this purpose, according to the invention, after the natural frequency is once determined, the course of the amplification curve is still further influenced. This can be effected, as was indicatechabove, by varying the number of turns of the primary winding and consequently its impedance. If the impedance be increased, as compared with the usual adaptation to the impedance of the line, the relative value of the terminal voltages is reduced and with it the ratio of amplification of two frequencies lying on one side of the amplification curve. If, however, a smaller number of turns be used than that given by the adaptation, the ratio of amplification for the two frequencies becomes greater. Hence, by selecting a smaller number of turns than corresponds to the adaptation to the line, the amplification curve can be made steeper without additional electrical means, and it can be flattened by a greater number of turns.

In the accompanying drawin s. I have illustrated my invention grap ically by curves and dia rammatically by a circuit arrangement. I n these drawings Figure 1 represents raphs showing the course of the amplification curve under different conditions;

(winding from the value o, to 10 coil of the input transformer is enpediente,

Figure 2 shows a wiring diagram of an amplifier circuit embodying the features of the invention; and

Figure 3 shows a wiring diagram of a 'Inodification of the input circuit arrangement.

Referring to the curve diagram Figure 1, it will be noted that by raising the natural frequency of the secondary transformer which latter value is assumed to represent a comparatively high audio frequency value, the entire range of speaking frequencies between al and m is located within the ascending branch of the amplification curve I). The damping curve a is shown in full lines. Now it will be noted that, as occurs in practice, the amplification curve b doesnot run in parallel to the damping curve a, but tends to converge with the latter towards the higher frequencies. Now if for instance the number of turns in the primary Winding of the input transformer should be increased, the curve I) tends to flatten out and to assume the form shown at 0. Thus the amplification curve will run substantially in parallel to'the damping curve as required. In case it should be observed that the damping curve a originally is steeper than. the amplification curve so that the two curves would tend to converge in the direction of the lower frequencies, the number of turns of the primary input winding should be reduced because by this expedient the amplification ratio between two frequencies located on the'same branch of the amplification curve is increased, whereby the amplification curve becomes steeper and thus would run in parallel to the steeper damping curve.

Referring to the wiring diagram Figure 2, 1 represents a microphone, 2 an input transformer, whose primary coil as shown is variable and whose secondary coil is located in the grid circuit of the thermionictube amplifier 6 which contains the conventional filament F, grid G and plate P. In parallel to this secondary coil is placed a variable condenser 4 which serves for adjustin the natural frequency of the secondary coil to the value deslred. The output transformer 3 is connected with the late and filament'of the amplifier tube and the secondary coil of this transformer is connected to the output line 7. The filament is heated by the battery A, and a B-battery is shown in the plate circuit in the conventional manner, and a id biasing battery C is-shown in circuit with the filament grid circuit.

' Aocordin to the invention the adjusted so that its applarent resistance becomes smaller or larger t on that of the microphone, co-

the conditions of the line. lit by the complete diatinctnees in the speech tranamieoion in not obtained,

cording to of the amplification primary v this defect may be remedied by adjusting the natural frequency of the secondary coil of transformer 2 by means of condenser 4 as described hereinbefore. An inductance 5 is shown in the input circuit leading to the primary coil of the primary transformer, which may serve for controlling the course curve by influencing the phase angle between the line and the transformer. Instead of an inductance 5, which is placed in series with the input line, a capacity may be used if required, and either of these impedances may be placed in series with the input line or in parallel thereto. For instance in, Figure 3 a condenser 8 is shown in parallel to the input line.

In selecting the impedance of the primary coil the phase angle between the line and the impedance of the primary winding, which varies with the frequency, plays a part and must be taken into account. According to the invention the phase displacement can also be used for obtaining a greater approximation of the amplifying curve to the damping curve of the line. As stated above, this ma be accomplished by the insertion of a sel induction or capacity in series with the line or parallel to it as shown in Figures 2 and 3. I

Through a single variable electrical means, as alluded to above, it is of course not possible to fulfil all requirements that ma occur with regard to the flattening 0 the curve of acoustic frequency. By selecting the proper natural frequency of the secondary winding the osition of the amplifyihg maximum is fixe tion of the impedance of the primary winding the rise of the amplification curve is determined. It is, however, not possible to influence the character of the intermediate course of the curve by means of these two variables in the requisite manner. The course, of the amplification curve may still show deviations from the damping curve of the line. This is not when only one amplifier is used in the line. Where, however, several amplifiers are inserted in a long line, the diflerences between the damping and amplification curves will add up for the diderent amplifying arrangements, so that finally there would be a distortion which would be found very disturbing.

The required approximation of the am- Elification curve to the damping curve can e efi'ected successfully when the amplification curves of the individual amplifiers are no longer selected so as to beuniform, but so that they supplement each other. Through a difierent selection of the natural frequency for the individual amplifier units in conjunction with adapting the individual input transformers to values above or below the required value, the effect is obtained that and through the selecdisturbing in telephony,

tit

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the course of the resultant amplification curve of all the amplifiers approximates, to the required extent, the configuration of the damping curve of the whole line, in which the amplifiers are connected. In this case both, the distortion due to the damping of the line and the damping due to the transformers or other electrical means connected in the line must be taken into account.

The individual transformers are preferably not to be selected so that the natural frequency of the secondary winding has of itself the required value, but it is far better to so construct them that the natural frequencies of all transformers lie somewhat above the highest required natural frequency. It is then possible to decrease this natural frequency, as required, by suitable electrical means, more particularly by condensers which are connected in parallel with the primary or secondary winding, as shown in FiguresQ and 3. This entails the advantage that the manufacture of the transformers becomes less difficult. Transformers such as are now manufactured for a certain natural frequency, hardly ever turn out alike. Should the natural frequenc of the secondary winding of the finishe transformer lie below that which is required, the transformer cannot be used, while, if the secondary winding be designed so that its natural frequency lies higher than is required, it is easy to bring the natural frequency to the required value in the manner described above. The number of faulty transformers is thus reduced to a minimum.

What I claim is 1. A telephone installation comprising in combination a line, an amplifier and audio frequency input and output transformers connected with said amplifier and suitably dimensioned to have the natural frequency of the secondary coil of the input transformer lying within the range of higher audible frequencies and means for correcting the course of the amplification curve so as to bring its course into approximation with the damping curve of the line for the purpose of compensating the damping curve of the line by the amplification curve and of 0btaining a uniform remaining damping or attenuation of all those frequencies which are important for the transmission of speech.

2. A telephone installation comprising in combination a line, amplifiers and audio frequency input and output transformers suitably dimensioned to have the natural frequenc of the secondary coil of the input transormer lying within the range of higher audible frequencies and means for correcting the course of the amplification curve so as to bring its course into approximation with the damping curve of the line, comprising instrumentalities for selecting the impedance for the primary coil of the input transformer, said impedance being dependent on the frequency, for the purpose of compensating the damping curve of the line by the amplification curve and for obtaining a uniform remaining damping or attenuation of all those frequencies which are important for the transmission of speech.

3. A telephone installation comprising in combination a line, amplifiers and audio frequency input and output transformers, suitably dimensioned to have the natural frequency of the secondary coil of the input transformcr lying within the range of higher audible frequencies and means for correcting the course of the amplification curve so as to bring its course into approximation with the damping curve of the line, comprising instrumentalities for varying the number of turns of the primary coil for the inputtransformer. said impedance being dependent on the frequency, for the purpose of compensating the damping curve of the line by the amplification curve and for obtaining a uniform remaining damping or attenuation of all those frequencies which areiiuportant for the transmission of speech.

4. A telephone installation comprising in combination a line, an amplifier and an audio frequency input and output transformer connected with said amplifier to transmit a band of frequencies, the secondary Winding of the input transformer being required to resonate at a frequency within said band to give efficient transmission for frequencies in the upper range of said band, the natural frequency of the secondary winding of the input transformer being originally higher than is necessary for the purpose required, and means for decreasing said original frequency to the natural frequency required by the articiilar line condition, said means comprising a condenser connected in parallel with one of the two windings of the input transformer.

5. A telephone installation comprising in combination a line, a plurality of amplifiers and a corresponding plurality of input and output transformers connected to said amplifiers and being suitably dimensioned to have the natural frequency of the secondary coil of each input transformer lying within the range of higher audible frequencies and means for correcting the course of the amplification curve of each amplifier so as to bring its course into approximation with the damping curve of the line for the purpose of compensating the damping curve of the line by the amplification curve and of obtaining a uniform remaining damping or attenuation of all those frequencies which are important for the transmission of speech, the amplifying curves of the individual amplifiers also being determined by aforesaid means relatively to each other so that the resultant amplification curve of all amplitiers substantially has the configuration of the damping curve of the line.

6. A telephone installation comprising in combination a line, an amplifier, an audio frequency transformer having primary and secondary windingsassociating said amplifier with said line, and means for compensating for the varying attenuation of said line with frequency, said means comprising a condenser in parallel with the secondary winding of said transformer and of such a value as to make the natural frequency of said secondary winding lie Within the range of the higher audible frequencies, and an inductance element in series with said primary winding.

7. In combination, a line, an amplifier, an audio frequency transformer having primary and secondary windings associating said amplifier with said line, and a resonant circuit comprising an impedance in shunt to the secondary winding of said transformer Lea-5,778;

and an impedance efiectively in series with the primary winding of said transformer, said shunt impedance and said series impedance having such relative values as to produce an amplification characteristic for said transformercomplementary to the attenuation characteristic of said line.

8. A telephone installation comprising in combination, a line, an amplifier, an audio frequency transformer having primary and secondary windings connecting said amplifier' with said line, and means for producing an amplification characteristic for said transformer complementary to the attenuation characteristic of said line, said means comprising a condenser in shunt to the secondary winding of said transformer and an impedance element in series with the primary winding of said transformer.

In testimony whereof I affix my signature.

BRUNO GERHARD POHLMANN. 

